1. Field of the Invention
The present invention relates to a hot runner system for molding preforms, and more particularly to a hot runner system which is capable of simultaneously molding a plurality of preforms per injection packing of a resin.
2. Background Art
Thin containers such as PET (polyethylene terephthalate) bottles are molded by subjecting injection-molded preforms to stretch blowing in a blow mold. Further, a plurality of preforms are injection-molded per one molding operation, and therefore a so-called hot runner system is employed as an injection molding machine for molding preforms. Generally, the hot runner system is comprised of a runner block, a gate formed on a side surface of the runner block at a lengthwise central portion thereof, and a plurality of nozzle members protruding from respective upper portions of the runner block. The runner block has a hot runner internally extending through a core portion thereof, such that the hot runner communicates between the gate and each of the nozzle members. The runner block is supported at the lengthwise central portion thereof and both end portions thereof by means of support plates standing on a base plate.
The runner block has its periphery enclosed by band heaters which function to uniformly heat the entire runner block, to thereby keep the temperature of the molten resin contained in the hot runner to a constant value. In addition, additional band heaters are attached to peripheries of the nozzle members, the gate member, etc. in order to uniform the molten resin and prevent its temperature from decreasing. However, due to its mechanical structure, the temperatures at the lengthwise central portion and the end portions of the block runner show a tendency to decrease.
This tendency is considered to be caused by heat loss due to heat transfer to the support plates supporting the lengthwise central portion and the end portions of the runner block as well as heat radiation at the end portions of the runner block. The heat transfer to the central support plate can be suppressed relative to the heat transfer to both the end support plates because of heat from the injection device transferred by nozzle touch. On the other hand, at the end portions of the runner block, the temperature of the resin flowing through the hot runner decreases and become non-uniform, which disadvantageously results in one-sided wall of preforms made from the resin injected from the nozzle members at the end portions which are located at the farthest locations from the gate.
This disadvantage can be overcome by interposing heat insulating materials between the runner block and the respective support plates, and further by covering both end surfaces with heat insulating materials. However, this can cause another disadvantage. That is, each nozzle member abuts a cavity gate of the injection mold for molding preforms, and therefore high accuracy is required for fitting the support plates to the runner block. In addition, compressing force by the injection device due to the nozzle touch is supported by both fitting portions between the runner block and the support plates, at the end portions of the runner block. If the heat insulating materials are interposed, the fitting portions can become deformative, which damages horizontality of the runner block, and therefore the nozzle touch with the entire cavity gates cannot be appropriately performed, resulting in resin leakage. Further, prevention of the heat radiation by covering the runner block, etc. with the heat insulating materials is difficult from the standpoint of the mechanical structure. Therefore, it is difficult to overcome the disadvantage by employing the heat insulating materials.
It is therefore an object of the present invention to provide a hot runner system for molding preforms, which is new in construction and capable of overcoming a disadvantage of heat loss at both end portions of the runner block, by heating both the end portions per se.
To attain the object, the present invention provides a hot runner system for molding preforms, being arranged under a preform-molding injection mold having cavity gates, the hot runner system including a plurality of nozzle members abutting the cavity gates of the injection mold, respectively, a cylindrical runner block having a gate formed on a lengthwise central portion of a side surface thereof, the runner block mounting therein the plurality of nozzle members such that the nozzle members protrude from respective upper portions of the runner block, the runner block having a hot runner extending through a core potion thereof to communicate between the gate and an interior of each of the plurality of nozzle members, the runner block having band heaters enclosing a periphery thereof, blocking members fitted into both end portions of the runner block, for blocking the hot runner, and support plates fitted to the runner block at both the end portions of the runner block, the hot runner system comprising a heater inserted into a core portion of each of the blocking members and a heater enclosing a periphery of the each of the blocking members, for compensating for heat loss due to heat radiation and heat transfer to the support plates, to thereby keep temperature at the each of the end portions of the runner block in a predetermined range of temperature.
Preferably, the each of the blocking members is of an inside plug shape and formed by a lid body which is fitted into a concave portion formed in an end surface of the runner block, and a shaft portion which is fitted into a runner opening which is larger in diameter than the hot runner, and wherein the heaters for compensating for the heat loss comprise a cartridge heater embedded in the shaft portion and having a length from an external end surface of the lid body to a portion close to an end portion of the hot runner, and a heating plate tightly attached to the external end surface of the lid body together with a second band heater enclosing the same.
In the construction described as above, the internal and external heaters provided for each of the blocking members function as heating members at the respective end portions of the runner block. As a result, the heat loss caused by the heat radiation and the heat transfer at both the end portions can be eliminated, to thereby prevent non-uniformity in temperature at both the end portions of the runner block due to temperature decrease. Therefore, heating by the band heaters enclosing the periphery of the runner block can be uniformly kept in the entire runner block, to thereby keep the temperature of the entire runner block in the predetermined range. In this manner, it is prevented that preforms are one-sided due to the temperature decrease of a resin injected from the nozzle members arranged at both the end portions of the runner block.